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Project: Onkyo TX-8500 (mkI)

Dang, late to the party. Maybe other approach could spare you the hassle of making the dedicated PCB for filter capacitors. You did an amazing job, I have restored three of these (including the one I own) and it is definitely not for faint-hearted.


upload_2021-11-20_22-5-10.png Dual caps final.jpg
 
I saw you approach…. very nice. The filter board was just an attempt to challenge myself to design and build a solution. Am very happy with the compact solution that incorporates increased capacitance, bypass caps, and bleed resistors.
 
Your filter cap solution is one of the best I've seen! I am starting a 8500 restoration soon and wonder if you have extra boards or can share the pcb file you made? Many thanks, John
 
Hi, I love your walk through. Thanks for sharing. Back in 2017 I completed 98% of a restoration of my TX-8500. I got stuck on the those 2-in-1 filter caps and shelved it in frustration :) Now I'm back at it. Would you be willing to share the board specifications so I can finish my project? Or perhaps you could point me to the xRayTony video that you referenced. I searched through his videos but could not tell which one involved this custom board.
 
@Super Noob I know it's an old thread, but is there a typo in capacitors spec for the AMP Assemblies (NADA 405)?
Capacitors:
C03 47 uF 16v Electro 47 uF 25v UPW1H101MPD
C04 47 uF 10v Electro 47 uF 16v UPW1H101MDD
C17 10 uF 63v Electro 10 uF 100v UPW2A101MHD
C18 10 uF 63v Electro 10 uF 100v UPW2A101MHD

Both UPW2A101MHD and UPW1H101MPD are listed as 100 uF capacitors on Mouser.
Is it a typo in the model name (should really be a 10 uF capacitor), or did you replace them with 100 uF capacitors?
 
@Super Noob How did you arrive with a value of 15 mV to set the bias current on the amplifier boards (NADA-405)?
I'm just curious and don't remember how to calculate it properly. The specs are 40-50 mV, but none of the resistors were changed except for the trim pots.

BTW, I had to play with the J1-J2 jumpers to bring the center voltage closer to zero. Mine was 30 mV off in one channel and about 20 mV in the other.
Thanks for the guide!
 
Mico said:
Onkyo adjustment procedures are one of the muddiest around. I got simplified version for my TX-8500 few years ago, let me check and I will post it here. For now, 40mV idle current as per SM procedure is too high . Very difficult to adjust DC offset, no variable resistor and that jumper gimmick was never clarified to me. Just be careful, it is quite easy to short something there...
EDIT; that is what I got for idle current and was told to leave the rest alone:
Disregard the adjustment procedure. Measure the voltage across R39. Use R22 and adjust it for 10mV. Do the same for the other channel.
Advice comes from one of the engineers who designed the receiver. When I asked why SM was so confusing he answered, " I did not write it " :).
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Across both those .47 ohm emitter resistors should be 16 ma
 
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Interesting. The original spec 40 mV between ID and CT translates then to the same voltage drop on R39 and R41, which means 40/0.47= 85 mA idle through each of the output transistors Q12 and Q14... Maybe they cranked it up to make sure it meets the THD specs with a margin?
 
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invariant said:
Interesting. The original spec 40 mV between ID and CT translates then to the same voltage drop on R39 and R41, which means 40/0.47= 85 mA idle through each of the output transistors Q12 and Q14... Maybe they cranked it up to make sure it meets the THD specs with a margin?
Click to expand...
That is correct, measuring across R39 or ID and CT points would give the same idle current value. Service manual calling for 40mV is too high and would make receiver run warmer than needed. Which was probably the case when these were brand new. But 10mV recommended by Onkyo engineer was also absolute minimum. I have this one since 2014 (fully restored and recapped) and had a chance to observe the bias shift over time. I was always a bit skeptical about this 10mV recommendation and instead went for 15mV, and by 2018 is slipped to 8-9mV, almost 50% in just 4 years! And this drop did affect the sound, big time. Only when another 8500 arrived for restoration I realized something was off with mine. So I readjusted to 25mV and to this day (6 years) it dropped just 3mV. As a side note, I had a chance to work on another two 8500 (I) receivers, both being 100% original and intact. All of them, including mine before restoration, had a factory bias between 25-32mV. If we assume these were factory adjusted as per service manual (40mV) that shows the bias on this unit will most likely go down over time. I recently readjusted again to 25mV and will revisit it sometime in 2030 ;-)
 
Super Noob said:
In my quest to NOT buy anymore 70s receivers / amplifiers.... I have relapsed again. My focus is limited = great value, typical larger WPC, not working. Have read a lot about the TX-8500 mkII Onkyo and wished to find one. A TX-8500 mkI popped up locally in non-working state = it must be fate.

Here is how she landed in the garage:
AM-JKLW6iTNa2RIptGg8tqYgFc0C8UBjQnVALtHYWTh533QYn7sBygZGTF14YChoGHgcoLJLfypmsOrjjzzhbpECbe4lYp5iJHCFHaFKQ56IEmN_9iFMu78MyoymmZi3K21_QhW4yR8ia5YmEYWCOt8-kXCh=w876-h657-no


I suffer from too much gear... I find / I fix / I keep (maybe could be called hoard). I have moved plenty of gear on to friends and audio enthusiasts. When you have the time I have in these rebuilds, it is hard not to become attached to the gear. This rig will be gifted to my good friend "Tony" in the panhandle.

From what I have read online looks like Tom Ishimoto was involved with this rig and was available around 1978. The specs show 110 WPC, 8 ohm, .1% THD, 20 Hz to 20 kHz.

giphy.gif


Many posts to follow walking through the rebuild / restoration. This is my hobby, and I like to document builds to help or motivate others in the future. Hope you enjoy the thread.

Don't forget that I am just a NOOB with a soldering iron and the internet.
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Super Noob, Really enjoy seeming / reading refurb projects like this! When you recapped (EC's only, pic looks like all EC's) the TX-8500 mkI fm/tuner board did it require any retuning or fm adjustments? If so, what was done? Thx, Walt
 
Super Noob, you are absolutely a super technician and I can't thank you enough for all of your pictures and writings about your restoring this TX-8500. I have one I bought for next to nothing and I have tried to work on the right power amp board because the right channel is out ---- even replacing the output transistors, but no luck yet. Thanks again for posting everything to did.
 
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